1. Field of the Invention
The present invention relates to an improvement in an auto-pedestal level clamp circuit in an automatic luminance control circuit of a television.
2. Description of the Prior Art
In an automatic luminance control circuit of a television, since pedestal level varies depending on the video signal, an auto-pedestal level clamp circuit is adopted to forcibly clamp the pedestal level at a given level.
FIG. 1 shows a conventional auto-pedestal clamp circuit. Referring to FIG. 1, reference numeral 1 denotes a control for generating a divided voltage of a positive power source voltage (to be referred to as V.sub.DD hereinafter) and a negative power source voltage (to be referred to as V.sub.SS hereinafter); 2, a diode; 4, 5, 9, and 10, capacitors; 3, 6, and 8, resistors; 7, a transistor; 11, a pedestal level clamp switch which is turned on or off in response to a pedestal level clamp pulse; and 12, an A/D converter for quantizing the composite video signal.
Referring to FIG. 1, the diode 2 connected to an intermediate tap of the control 1 clamps a composite video signal passing through the capacitor 5, and the clamped signal is smoothed by the resistor 3 and the capacitor 4. The collector potential of the transistor 7 varies depending on the change in smoothed potential, thus charging or discharging the capacitor 9. More specifically, in a bright frame (i.e., when a luminance signal in the composite video signal is high), since the base potential of the transistor 7 increases, the collector current increases accordingly, and the capacitor 9 discharges, thus decreasing the potential of the capacitor 9. On the other hand, in a dark frame (when the luminance signal is low), the capacitor 9 is charged, and its potential increases.
FIGS. 2(A) to 2(C) are timing charts showing the relationship between the pedestal level clamp pulse and the ON/OFF operation of the pedestal level clamp switch with respect to the composite video signal during about 2H (H: horizontal period) of the composite video signal. FIG. 2(A) shows the composite video signal, and FIG. 2(B) shows the pedestal clamp pulse which goes to HIGH level during an interval from the leading edge of a horizontal sync signal to the luminance signal in the composite video signal from which DC components are removed (this interval will be referred to as a back porch hereinafter).
Referring to FIG. 2(C), "close" indicates the state wherein the two ends of the pedestal level clamp switch are enabled, and "open" indicates the state wherein they are disabled. Since the "close" timing coincides with the HIGH level interval of the pedestal clamp pulse in FIG. 2(B), charges are accumulated on or discharged from a DC blocking capacitor for the composite video signal, so that the voltage during the back porch (to be referred to as a pedestal level hereinafter) is equal to that at the other end of the pedestal level clamp switch.
For example, when image contrast is low as in the time-division drive display of a liquid-crystal television, multi-gradation display is not easily realized. In addition, when the dynamic range of gradation display is adjusted in a bright frame, this results in poor gradation in a dark frame. For this reason, the auto-pedestal level clamp circuit is necessary for enabling averaged display by narrowing the dynamic range determined by an A/D converter and the like, so that the pedestal level is decreased in a bright frame to display many bright image portions and increased in a dark frame to display many dark image portions.
However, in order to obtain a satisfactory display frame with the circuit shown in FIG. 1, the control 1 in FIG. 1 must be finely adjusted, resulting in increased number of steps and hence increased cost. In FIG. 1, since the capacitors 4, 5, and 9 have a capacitance of 0.1 to several tens of .mu.F, they cannot be integrated as an IC and must be separately arranged, thus interfering with cost reduction and compact circuit design. In addition, since the output from the A/D converter in FIG. 1 is not fed back for setting the pedestal level, a time constant for circuit constituents must be accurately selected, thus narrowing the application range with respect to the amplitude of the composite video signal.